U.S. Pat. No. 5,944,197 describes woven wire screencloth having a plurality of parallel warp filaments intersected by a plurality of parallel weft filaments configured with a greater number of warp filaments than weft filaments per given area so as to form rectangular openings in the weave. Each opening therefore has a length and width dimension and cloth having such rectangular openings has been found to possess certain desirable properties in relation to the filtering of certain muds.
According to U.S. Pat. No. 5,944,197 the length to the width ratio of each rectangular opening is optimally in the range 2.7 to 2.8 and an optimal ratio of the length of each opening to the diameter of the weft filaments has been found to lie in the range 5.5 to 5.7. This US Patent also teaches that in order to take advantage of the rectangular openings in the mesh, the interstices must be locked to prevent the warp and weft wires from slipping during operation. Therefore by calendering the wire cloth between a set of rollers, the filaments are compressed at the intersections of the warp and weft and this assists to resist movement between the warp and weft filaments. Combining optimal wire diameter ratio and calendering, permitted an increased length to width ratio to be employed in the cloth.
Ideally screen conductance is optimised and in general this occurs when wire diameters for the warp and weft filaments are as small as is practical. However the desire for increased conductance can have a negative impact on the quality of the screen and its effectiveness and using smaller diameter warp and weft filaments can result in the warp and weft not remaining perpendicular. Furthermore reducing wire diameter can reduce useful screen life due to wear and tear and breakage of the small diameter wires. The mesh provided by the teaching of U.S. Pat. No. 5,944,197 has been shown to have a greater conductance and improved life over prior art meshes when used to make similar screens, since by employing slightly larger diameter wires but omitting some of the weft wires, so as to produce rectangular openings in the mesh, there is a trade-off and the reduced conductance is offset by the increased screen life relative to prior art mesh constructions.
Correct calendering of the cloth assists in preserving the orthogonality of the warp and weft filaments and maintaining the high ratio of length to width in each of the rectangular openings in the weave. However insufficient calendering pressure fails to lock the warp and weft wires so that the orthogonality is compromised and the desirable opening ratios altered. Too much calendering pressure excessively deforms the wires resulting in shortened operational life. Consequently manufacture of the cloth described in U.S. Pat. No. 5,944,197 has to be the subject of tight tolerances and close control. Even correctly calendering the wire cloth deforms the warp and weft filaments which when other screen life-shortening conditions are present can result in earlier failure than otherwise might be the case.
The present invention seeks to provide a woven wire cloth screen in which the openings between warp and weft filaments are rectangular and the length to width ratios are maintained, thereby retaining the good conductance characteristics of such a cloth, but which does not require the cloth to be calendered.
Since the invention involves the use of wire cloth which is woven in accordance with the techniques described in U.S. Pat. No. 5,944,197 (albeit without being calendered), references to such cloth will be referred to as cloth of the type described. It is to be understood however that the reference to such cloth is to a wirecloth which has been produced without the step of calendering the cloth after weaving, and is a reference to the choice of warp and weft material, sizes, length to width ratios of the openings in the cloth, and opening length to weft filament diameter ratios.